Carbon fibre offers many advantages as a reinforcing material in that it generally has high mechanical strength and elasticity and low specific gravity, along with other excellent properties such as heat resistance, chemical resistance, electric conductivity, self-lubrication and the like. In producing a composite material (i.e., a fibre-reinforced plastic), a carbon fibre in the form of long or short fibre, or a woven fabric or a non-woven fabric is used in combination with a matrix of a synthetic resin. Resins suitable for such a matrix include thermoplastics resin such as polyacetals, polyamides, polyesters, acrylic resins, fluoro resins, polyvinylchloride resins and the like and thermosetting resins such as epoxies, phenolic, polyimides and the like. Such a composite material is versatile and may be used as a structural material, a heat-resistant material, an electrically-conductive material, or a friction resistant material. However, difficulties have been encountered in that carbon fibre has generally poor affinity for a synthetic resin, so that when mixed with a synthetic resin matrix, it is difficult to obtain a composite material having satisfactory physical or mechanical properties since the carbon fibre has low adhesion to or wettability with the matrix. Accordingly, it is common practice in the production of the composite material to pretreat the surfaces of a carbon fibre so as to improve the affinity of the carbon fibre for a synthetic resin. For example, in a well known oxidation method, the carbon fibre is oxidized in a liquid or gas medium to form oxygen-containing functional groups such as carboxyl groups, carbonyl groups, lactone groups and/or hydroxyl groups on the surface of the fibre and at the same time to increase the surface area of the carbon fibre. However, the oxidized carbon fibre is still deficient in affinity for synthetic resin and the resultant composite material using the oxidized carbon fibre is disadvantageously low in resistance to water since the functional groups formed on the surfaces of the carbon fibre are hydrophilic groups. Furthermore, the carbon fibre having surfaces thereof more or less acidified by the produced functional groups may attack a synthetic resin matrix during the production of a composite material. More specifically, where a polyacetal resin which is susceptible to attack by an acid is used as a matrix, the matrix resin and the ultimate composite product has reduced physical or mechanical properties.
Accordingly, there has been a strong demand for a method of treating surfaces of a carbon fibre to impart thereto an improved affinity for a synthetic resin matrix and excellent adhesion to the matrix, without adversely affecting the resultant composite material obtained therefrom.